This application claims the benefit of Japanese Patent applications Nos. 2001-024540 and 2001-024558 which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a variable focal length lens system and, in particular, relates to a variable focal length lens system suitable for a high zooming ratio, and a small-sized variable focal length lens system capable of near distance focusing with a small number of lens elements at a high zooming ratio.
2. Related Background Art
Excellent portability is regarded as important for a lens shutter type camera by the users. In order to achieve such excellent portability, that is, to obtain a camera with excellently reduced weight and size, it is important to reduce the total size of a picture-taking optical system, and the thickness and the diameter of each lens element.
A lens shutter type camera generally employs a zoom lens as the picture-taking optical system. The zoom lens is adapted to enlarge an image which is formed by means of a lens group disposed closer to the object side than this zoom lens, in a refractive power layout of a so-called telephoto type in which a negative lens group is disposed on the most image surface side. It is possible to achieve a predetermined zooming ratio by moving the negative lens group in such a manner that a magnification change ratio becomes small in a wide angle end state and becomes large in a telephoto end state when the positional state of the lens is changed from the wide angle end state to the telephoto end state. Simultaneously, an aperture stop is disposed in such a manner that a distance between the aperture stop and the negative lens group is wide in the wide angle end state, and is gradually decreased to follow a change in the positional state of the lens from the wide angle end state to the telephoto end state, so as to be narrowest in the telephoto end state. Consequently, in the telephoto end state, an off-axis light flux passes away from the optical axis. Also, the closer the lens position comes to the telephoto end state, the closer the off-axis light flux comes to the optical axis. By arranging the lens system as described above, it is possible to satisfactorily correct variations of off-axial aberrations which are caused by changes of the lens positional state.
In these picture-taking optical systems, since the distance between the negative lens group and the aperture stop is wide in the telephoto end state, the lens diameter of the negative lens group tends to be large. Consequently, it is important to reduce the lens diameter of the negative lens group, in order to reduce the lens barrel size, and to further reduce the height and the width of the camera body.
A zoom lens with a high zooming ratio is generally provided with three or more movable lens groups, which is a so-called multi-group zoom lens system. As such a lens system, a positive/positive/negative three-group type zoom lens system which has a comparatively small number of variable lens groups is mainly used. This positive-positive-negative three-group zoom lens system is composed of three lens groups, which includes a positive lens group, a positive lens group and a negative lens group, from the object side in this order.
For example, in a zoom lens system disclosed in Japanese Patent Application Laid-Open No. 2000-66100, a negative lens group is composed of three lenses including a positive lens with the convex surface facing the image side, a negative lens with the concave surface facing the object side and a negative lens with the concave surface facing the object side from the object side in this order.
Also, in a lens system disclosed in Japanese Patent Application Laid-Open No. 2000-56223, a negative lens group is composed of two lenses which include a positive lens with the convex surface facing the image side and a negative lens with the concave surface facing the object side from the object side in this order. In this lens system, the lens surface of the positive lens on the object side is formed aspherical. With this arrangement, it is possible to increase the degree of freedom in correcting aberrations and, instead, reduce the number of the lenses by one.
Further, in a lens system disclosed in Japanese Patent Application Laid-Open No. 2000-155263, a positive lens in a negative lens group is formed as a double aspherical lens. With this arrangement, it is possible to increase the degree of freedom in correcting aberrations and to enhance the performance of the lens system.
In order to reduce a lens diameter, it is the most appropriate to approximate an off-axis light flux reaching a negative lens to the optical axis with strong convergence effect of a positive lens. However, when the convergent effect of the positive lens is enhanced, a negative spherical aberration is generated in a large amount. For this reason, the convergence effect in the periphery of the optical axis is increased, compared with around the optical axis with introduction of an aspherical surface, so as to enhance the refractive power and to reduce the lens diameter.
As it becomes common to provide a zoom lens system as an picture-taking optical system, in order to enhance a zooming ratio, such cameras of a type in which the focal length thereof is large in a telephoto end state have been widely spread. The zooming ratio in this case equals to the quotient which is obtained by dividing the focal length in the telephoto end state by the focal length in the wide angle end state. With the increase of the focal length in the telephoto end state, the total length of the lens system becomes large, which results in poor portability. Accordingly, when carrying a camera, a zoom lens system is accommodated in the camera body in such a manner that spaces between the lens groups are reduced to the minimum, so as to improve the portability.
As a conventional zoom lens, a zoom lens, for example, disclosed in Japanese Patent Application Laid-Open No. 56-128911 is known. This zoom lens is of a two-group zoom type of positive and negative lenses, composed of a first lens group having a positive refractive power and a second lens group having a negative refractive power from the object side in this order. In this zoom lens type, the respective lens groups are moved toward the object side in such a manner that a distance between the first lens group and the second lens group is reduced when the focal length is changed from the wide angle end state to the telephoto end state, so as to change the focal length. However, when a zooming ratio is increased by 2.5 times or more, the total length of the lens system in the telephoto end state becomes extremely large. For this reason, there is proposed a zoom type of positive-positive-negative three groups, which has an additional variable lens group.
A lens of this three group zoom type is disclosed, for example, in Japanese Patent Application Laid-Open No. 2-16515, The disclosed lens system is of a positive-positive-negative three group zoom type, which comprises a first lens group having a positive refractive power, a second lens group having a positive refractive power and a third lens group having a negative refractive power from the object side in this order. In this zoom type lens, the respective lens groups are moved toward the object side in such a manner that a distance between the first lens group and the second lens group is increased and a distance between the second lens group and the third lens group is decreased when the focal length is changed from the wide angle end state to the telephoto end state, so as to change the focal length. With the increase of the number of the variable lens groups as described above, it is possible to reduce the total length of the lens system even if a zooming ratio is increased.
On the other hand, there is known a near distance focusing method as disclosed, for example, in Japanese Patent Application Laid-Open No. 7-151953. By this method, a zooming operation and a focusing operation can be conducted with a single driving source. In accordance with an angle of rotation to be given to a zoom ring, the lens positional state is changed from a wide angle end state (infinity focusing), to another wide angle end state (near distance focusing), to a telephoto end state (infinity focusing), and to another telephoto end state (near distance focusing), and the first lens group is arranged to be moved toward the object side accordingly.
As described above, in the lens system disclosed in Japanese Patent Application Laid-Open No. 2000-56223, the lens surface on the object side of the positive lens takes an aspherical form. This aspherical form effects divergence around the optical axis, and convergence in the peripheral area. In the vicinity of the optical axis, the concave surface of the lens faces the aperture stop to smoothly refract an off-axis light flux. On the other hand, in the peripheral area, the convex surface of the lens faces the aperture stop so that the light flux is rapidly refracted. For this reason, the performance of the lens system may be abruptly deteriorated even by a slight decentration.
Also, in the lens system disclosed in Japanese Patent Application Laid-Open No. 2000-155263, the positive lens is formed as a double aspherical lens, as described above. With this arrangement, the convergence effect of the lens surface on the image side becomes stronger in the peripheral area. For this reason, the deterioration of the performance due to a slight decentration can be satisfactorily suppressed. However, since the radius of curvature in the peripheral area is rapidly decreased, it is difficult to process the aspherical lens.
With the increase of the focal length in the telephoto end state, the total length of the lens system in the telephoto end state becomes large. For this reason, the size of the lens barrel also becomes large. In addition, the length of each of the barrel elements for constituting the lens barrel also becomes large, so as to cause inconveniences to the portability. Accordingly, it conceivable that the thickness of the camera in its accommodated state is reduced, by increasing the number of the barrel elements and reducing the length of each barrel element. However, on the contrary, the diameter of the lens barrel becomes large, and the height and the width of the camera body become large, too. As a result, the portability is spoiled.
Furthermore, it is conceivable to avoid inconveniences to the portability by reducing the total length of the lens system in the telephoto end state. In order to reduce the total length of the lens system, it is possible, for example, to reinforce the refractive power of each lens group, or to increase the number of movable lens groups. However, in the former case, the accuracy in stopping each lens group may be extremely high, or the performance of the lens system may be greatly deteriorated. In the latter case, the number of lenses becomes large, which is against the object of the present invention for achieving a light-weight and compact variable focal length lens system with a smaller number of lens elements.
According to the method of near distance focusing disclosed in Japanese Patent Application Laid-Open No. 7-151953, in the telephoto end state at a near distance focusing, the first lens group is fed out toward the object side. For this reason, the total length of the lens system in this case becomes larger than that of the lens system in the telephoto end state at an infinity focusing.
An object of a first aspect of the invention is to provide a variable focal length lens system of a reduced size and an increased zooming ratio in spite of a smaller number of lens elements for constituting the lens system.
In order to achieve the above object, according to the first aspect of the invention, there is provided a variable focal length lens system which comprises:
a first lens group having a positive refractive power, a second lens group having a positive refractive power, and a third lens group having a negative refractive power from the side of an object in this order,
wherein the respective lens groups are moved toward the object side in such a manner that a distance between said first lens group and said second lens group is increased and a distance between said second lens group and said third lens group is decreased when the lens positional state is changed from a wide angle end state to a telephoto end state;
an aperture stop is disposed in the vicinity of said second lens group and is moved together with said second lens group in accordance with a change of the lens positional state;
said third lens group is composed of a positive lens with the convex surface facing the image side and a negative lens disposed on the image side of said positive lens to have an air gap therebetween and have the concave surface facing the object side, said positive lens being a double aspherical lens and satisfying the following condition:
0.9 less than xcexaa(Ya/Ra)2 less than 1.3,
xe2x80x83where:
xcexaa is a conical constant of an image-side lens surface of the double aspherical lens disposed in said third lens group;
Ya is an amount defined by the following expression:
Ya=Ymaxxc2x7(DA/DS),
xe2x80x83where Ymax is a half of a vertical angle of the frame, DA is a distance from said image-side lens surface to the image surface in the wide angle end state, and DS is a distance between said aperture stop to said image surface in the wide angle end state; and
Ra is a paraxial radius of curvature of said image-side lens surface.
An object of a second aspect of the invention is to provide a small-sized variable focal length lens system with a high zooming ratio which is suitable for obtaining a high zooming ratio with a smaller number of lens elements and which is capable of effecting near distance focusing.
In order to achieve the above object, according to the second aspect of the present invention, there is provided a variable focal length lens system which is capable of a near distance focusing and comprises a first lens group having a positive refractive power, a second lens group having a positive refractive power, and a third lens group having a negative refractive power from the side of an object in this order, wherein the respective lens groups are moved toward the object side in such a manner that a distance between said first lens group and said second lens group is increased and a distance between said second lens group and said third lens group is decreased when the focal length is changed from a wide angle end state to a telephoto end state, which lens system being characterized in that:
in the wide angle end state, at least said first lens group is moved toward the object side when a near distance focusing operation is effected, while in the telephoto end state, at least said first lens group is moved toward the image side when a near distance focusing operation is effected, so as to satisfy the following conditions:
0.2 less than xcex33w less than 0.7,
and
1.1 less than xcex33t less than 1.5,
xe2x80x83where xcex33w is a ratio of an amount of movement (movement ratio) of the third lens group with respect to an amount of movement of the first lens group in the wide angle end state at the near distance focusing, and xcex33t is a ratio of an amount of movement (movement ratio) of the third lens group with respect to an amount of movement of the first lens group in the telephoto end state at the near distance focusing.